Supercharging a diesel engine which powers a large commercial vehicle such as a truck or bus can improve engine/vehicle fuel economy and performance. A turbocharger is commonly used for supercharging such an engine. Externally cooled, engine exhaust recirculation (commonly called EGR) is an effective technology for reducing oxides of nitrogen (NOx) in engine exhaust and may be useful in qualifying an engine design for compliance with certain exhaust emission requirements.
The possibility of increasing EGR rates beyond present-day levels can further reduce NOx in engine exhaust. Achieving increased EGR rates depends on the ability to create adequate pressure to drive EGR from an engine exhaust system to an engine intake system at various engine operating conditions.
When an engine is operating in certain areas of an operating map, such as a map of engine speed/engine torque or a map of engine speed/engine horsepower, pressure drop across the engine (i.e., the difference between exhaust manifold pressure and intake manifold pressure, sometimes referred to as ΔP, may not be sufficiently high to drive increased EGR.
If an engine has an intake throttle which is used for various purposes such as reducing air/fuel ratio or increasing exhaust temperature to assist exhaust after-treatment or regenerate a diesel particulate filter, the intake throttle can also be used to drive EGR under certain conditions by throttling intake flow into an intake manifold to increase ΔP. But doing that creates a fuel economy penalty because the engine is forced to work less efficiently.